1. Field of the Invention
This invention relates to ink jet printing devices and more particularly to a thermal ink jet or Microelectromechanical Systems (MEMS) printhead having an array of coplanar nozzles in a nozzle face that are entirely surrounded by an insulative polymeric material, together with a method of fabrication thereof.
Thermal ink jet printing is a type of drop-on-demand ink jet system wherein an ink jet printhead expels ink droplets on demand by the selective application of a current pulse to a thermal energy generator, usually a resistor, located in capillary-filled parallel ink channels a predetermined distance upstream from the channel nozzles or orifices. The channels' ends opposite the nozzles are in communication with an ink reservoir to which an external ink supply is connected. The current pulses momentarily vaporize the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it towards a recording medium. The printing system may be incorporated in either a carriage-type printer or pagewidth type printer. A carriage-type printer generally has a relatively small printhead containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge in a combined printhead and cartridge assembly which is reciprocated to print one swath of information at a time on a stationarily held recording medium such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. In contrast, the pagewidth printer has a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the printhead in a direction normal to the printhead length and at a constant speed during the printing process.
U.S. Pat. No. Re. 32,572 to Hawkins et al discloses a thermal ink jet printhead and method of fabrication. In this case, a plurality of printheads may be concurrently fabricated by forming a plurality of sets of heating elements with their individual addressing electrodes on one substrate, generally a silicon wafer, and etching corresponding sets of channel grooves with a common recess for each set of grooves in another silicon wafer. The wafer and substrate are aligned and bonded together so that each channel has a heating element. The individual printheads are obtained by milling away the unwanted silicon material to expose the addressing electrode terminals and then dicing the substrate to form separate printheads. This type of thermal ink jet printhead, where the direction of fluid ejection is substantially parallel to the plane of the wafer is sometimes called a sideshooter. A second generic type of ink jet printhead, called a roofshooter, has the direction of fluid ejection substantially perpendicular to the plane of the wafer. It is such roofshooter printheads that this invention applies to.
In microelectronics applications, there is a great need for low dielectric constant, high glass transition temperature, thermally stable, photopatternable polymers for use as interlayer dielectric layers which protect microelectronic circuitry. Poly(imides) are widely used to satisfy these needs; these materials, however, have disadvantageous characteristics such as relatively high water sorption and hydrolytic instability. There is thus a need for high performance polymers which can be effectively photopatterned and developed at high resolution.
Particular applications for such material include the fabrication of ink jet printheads as disclosed in related U.S. Pat. Nos. 5,762,812 and 6,260,956, the disclosures of which are incorporated herein. Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. One type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the “bubble jet” system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280° C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after about 20 to 500 microseconds minimum dwell time to enable the channel to refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known.
In ink jet printing, a printhead nozzleplate is provided having one or more ink-filled channels communicating with an ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. These printheads form images on a recording medium such as paper by expelling droplets of ink from the nozzles onto the recording medium. The ink forms a meniscus at each nozzle prior to being expelled in the form of a droplet. After a droplet is expelled, additional ink surges to the nozzle to reform the meniscus.
Roofshooting ink jet printheads include a nozzleplate having an array of nozzles. This nozzle plate may be bonded to a silicon wafer, for example, which contains the bubble nucleating heater elements
In U.S. Pat. No. 6,260,956 it has been proposed to use a polyarylene ether precursor polymer, which is photopatternable, to form the insulating layer over the heater plate, followed by photopatterning to expose the heating elements. The channel plate is prepared from the same photopatternable polymer and is then bonded to the heater plate using a thin bonding layer of the same polymer. This may be accomplished by indirect means in order to prevent the bonding layer from flowing onto the channel walls and along the apex of each channel, causing formation of a thin film along the channel walls and a bead along each apex.
It is desirable to provide a method for forming thermal ink jet nozzleplates by which a photopatternable resist layer can be applied to the patterned surface of an activator wafer, without disturbing said surface, and can be photopatterned to form ink nozzles having shapes which produce improved ejection velocity.
It is also desirable to provide a method for forming nozzleplates containing an ink cavity gap over a MEMS structure surface containing topography